U.S. patent number 4,893,161 [Application Number 07/100,795] was granted by the patent office on 1990-01-09 for quantum-well acoustic charge transport device.
This patent grant is currently assigned to United Technologies. Invention is credited to Sears W. Merritt, Robert N. Sacks, William J. Tanski.
United States Patent |
4,893,161 |
Tanski , et al. |
January 9, 1990 |
Quantum-well acoustic charge transport device
Abstract
An improved acoustic charge transport device having an acoustic
wave passing through a piezoelectric semiconductor is improved by
utilizing heterostructure Quantum-wells for confining charge
packets.
Inventors: |
Tanski; William J.
(Glastonbury, CT), Merritt; Sears W. (Glastonbury, CT),
Sacks; Robert N. (Glastonbury, CT) |
Assignee: |
United Technologies (Hartford,
CT)
|
Family
ID: |
22281578 |
Appl.
No.: |
07/100,795 |
Filed: |
September 24, 1987 |
Current U.S.
Class: |
257/183.1;
257/24; 257/245; 257/E29.189 |
Current CPC
Class: |
H01L
29/7371 (20130101); H03H 9/02976 (20130101) |
Current International
Class: |
H01L
29/66 (20060101); H01L 29/737 (20060101); H03H
9/02 (20060101); H01L 029/78 (); H01L 027/14 ();
H01L 029/205 (); H01L 045/00 () |
Field of
Search: |
;357/4,16,24,3E |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Quantum-well Charge-coupled Devices for Charge-coupled
Device-addressed Multiple-quantum-well Spatial Light Modulators",
by W. D. Goodhue, B. E. Burke, K. B. Nichols, G. M. Metze, and G.
D. Johnson 1 Nov. 1985, pp. 769-772, (J. Vac. Sci. Technol. B 4(3)
May./Jun. 1986. .
"Charge Transport by Surface Acoustic Waves in GaAs", by Michael J.
Hoskins, Hadis Morkoc, and Bill Hunsinger, 15 Apr. 1982, pp.
332-334, (Appl. Phys. Lett. 41(4), 15 Aug. 1982. .
"Monolithic GaAs Acoustic Charge Transport Devices", by Michael J.
Hoskins and Bill J. Hunsinger, pp. 456-460, (1982 Ultrasonics
Symposium)..
|
Primary Examiner: Munson; Gene M.
Attorney, Agent or Firm: Petraske; Eric W.
Government Interests
The Government has rights in this invention pursuant to Contract
No. F33615-86-C-1138 awarded to the Department of the Air Force.
Claims
We claim:
1. An integrated electronic acoustic charge transport device
comprising:
a substrate defining a horizontal plane;
charge transport layer of piezoelectric semiconductive material,
having a transport layer thickness and a transport layer conduction
band potential, disposed above said semiconducting substrate and
having a predetermined relationship thereto;
surface acoustic wave means disposed above an insulating support
means and mechanically connected to said charge transport layer,
for passing acoustic waves along a predetermined path in said
charge transport layer;
electron confinement means for preventing the escape of electrons
from said predetermined path;
electron supply means for supplying and maintaining a predetermined
concentration of supply electrons in the conduction band of said
charge transport layer, whereby at least some of said supply
electrons are carried along said predetermined path in electron
packets, having an average electron wavelength, by an electric
potential traveling with said acoustic wave; and
control means for controlling the passage of selected electron
packets along said path, characterized in that:
said charge transport layer has a first conduction band potential
and is positioned vertically between lower and upper confinement
semiconductor layers, each having a higher conduction band
potential than said transport layer conduction band potential;
and
said transport layer thickness is such that a potential well
extends through said transport layer, whereby said electron packets
are confined vertically in said charge transport layer by said
potential well and said transport layer thickness is less than 50
nm.
2. A device according to claim 1, in which said lower and upper
confinement semiconductor layer are formed of (Al,Ga)As; and said
charge transport layer is formed of GaAs.
3. A device according to claim 1, in which said lower confinement
semiconductor layer is formed from (Al,Ga)As, said charge transport
layer is formed from (In,Ga)As, and said upper confinement
semiconductor layer is formed from (Al,Ga)As.
Description
TECHNICAL FIELD
The field of the invention is that of SAW devices on GaAs, in
particular that of acoustical charge transport devices.
BACKGROUND ART
It is known to construct an acoustic charge transport delay line
using a SAW transducer to launch an acoustical wave through a
piezoelectric slab of semiconductor. FIG. 1 illustrates a sketch of
a prior art device. Note that a confinement electrode and
backgating is required in order to confine the charge packets in
the transport channel that are carried along by the acoustic wave
within the Gallium Arsenide (GaAs) slab.
It is also known in the art to use a charge-coupled device to
transport packets of charge down a delay line.
DISCLOSURE OF INVENTION
The invention relates to an improved acoustical charge transport
device in which an acoustic wave passing through a piezoelectric
semiconductor forms a series of potential wells that transport
packets of charge along with the wave. The improvements relate to a
superior confinement mechanism to form the transport channel to
confine the charge packets within the desired region.
The foregoing features and advantages of the present invention will
become more apparent in light of the following detailed description
of the best mode for carrying out the invention and in the
accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 illustrates a prior art device.
FIGS. 2a and 2b illustrate cross-sectional and plan views,
respectively, of a device constructed according to the
invention.
FIG. 3 illustrates an epitaxial layer structure that can also be
used.
BEST MODE FOR CARRYING OUT THE INVENTION
As was discussed above, a drawback with the prior art acoustic
charge transport device was the need for a set of electrodes and
corresponding power supplies and backgating to the epitaxial layer
in order to institute bias fields to confine the electrons within
the transport channel.
Referring now to FIG. 2a, there is shown in cross-section a
preferred embodiment of the invention. Substrate 10 is any
convenient material, illustratively GaAs. A first layer 20,
referred to as the lower trapping layer, follows along the
horizontal surface of the substrate and is formed from Aluminum
Gallium Arsenide (Al,Ga)As and has a thickness between 100 and
1,000 nanometers. Other materials for the trapping and transport
layers might be used and still be within the scope of the
invention. Above the lower trapping layer, there is a layer 25 of
GaAs, Indium Gallium Arsenide (In,Ga)As or other material,
illustratively 40 nanometers thick, that will be referred to as the
charge transport layer. It is through this layer that packets of
electrons are carried by the acoustic wave. Above GaAs or (In,Ga)As
layer 25 there is an upper trapping layer 30, also of (Al,Ga)As,
but n-doped. This upper trapping layer has an illustrative
thickness of 70 nanometers. The n-doping provides electrons to
satisfy traps associated with the free surface of the material. A
cap layer 35 is formed of GaAs.
A curve 28 is drawn superimposed on the cross-section in a
conventional manner to indicate the potential of the lowest level
of the conduction band. As can be seen, it drops sharply to a
bottom level within layer 25 and rises back again to form a channel
of lowered electronic potential. This conduction band structure has
formed a potential well based on the difference in conduction band
potential levels between (Al,Ga)As and GaAs. The thickness of the
GaAs layer 25 is chosen, among other reasons, to provide maximum
charge capacity by being thick enough to exceed the electron
wavelength in the channel. The optimum thickness is in the range of
20 to 40 nanometers. No benefit comes from thicknesses greater than
about 50 nm, because the mutual repulsion of the electrons produces
a concentration at the top and bottom surfaces. It will be readily
apparent to those skilled in the art that electrons somehow
injected into layer 25 will be confined vertically between layers
20 and 30 by the effect of the potential illustrated in the curve
28.
This illustrates a substantial improvement over the prior art
device illustrated in FIG. 1, since there is now no need for
backgating or to provide the prior art electrodes or the prior art
power supplies to bias the electrodes.
Lateral confinement of charge packets perpendicular to the plane of
the paper in FIG. 2a is provided by an area surrounding slab 25
that is rendered insulating, by conventional boron isolation, for
example. This area is indicated in the cross-section and in the
plan view of FIG. 2b by the numeral 100. Alternatively, other areas
may be rendered insulating by etching away the epitaxial layer,
thus placing the entire transport channel on an epitaxial mesa.
Referring now to FIG. 2b, there is shown a conventional SAW
transducer positioned on a section of insulating region 100 and
indicated by the numeral 40, that transforms an electrical RF
signal from generator 50 into an acoustic wave traveling through
the several layers shown in the cross-section in FIG. 2a. SAW
transducer 40 rests on an insulating area where the epitaxial layer
has been etched away. The wave length in GaAs at a frequency of
about 280MHz is approximately 10 microns and the bulk of the power
of the wave will be confined to the top layer of approximately 5
microns. There is thus an acoustical wave traveling from left to
right in FIG. 2a that contains alternate phases of compression and
rarefaction. Since GaAs is piezoelectric, these phases will produce
a corresponding electrical potential that travels along with the
acoustical wave. It will be evident to those skilled in the art
that there will be potential wells formed in layer 25. These wells
travel along with the acoustical wave, moving the charge packets
along with them.
The length of the delay line being formed by this device is bounded
by an input terminal and an output terminal, respectively referred
to by numerals 60 and 62, which are relatively heavily doped areas
sufficient to form an ohmic contact extending from the surface down
at least through layer 25. Electrons entering from electrode 64 of
the input section may travel freely down ohmic contact 60 to the
intersection between contact 60 and layer 25. When a potential well
passes through contact 60 traveling from left to right, electrons
are formed into packets and may flow from contact 60 into the right
hand portion of layer 25, carried along by the potential well. They
then travel through the length of layer 25 until they reach ohmic
contact 62 that provides an escape path out through electrode
66.
If terminals 64 and 66 are connected by a wire, current will flow
around the loop formed by the wire, ohmic contacts 60 and 62, and
charge transport layer 25. The motivating power to circulate
electrons around the loop is provided by the SAW wave, of course.
This steady train of charge packets may be controlled, for example
by electrode 70. Electrode 70 may be biased negative to create a
potential barrier that is high enough to prevent the force of the
SAW wave from carrying electrons past the barrier region. Thus,
pulses applied to electrode 70 can selectively block or permit the
passage of a charge packet. Information may thus be carried in
selected locations in the pulse train by controlling electrode 70.
The presence or absence of a charge packet in a selected region
will be noted by a corresponding voltage on any of the intermediate
electrodes 80. The device thus described therefore forms a tapped
delay line that can be used for a number of applications, as is
known to those skilled in the art.
In the preferred embodiment, the size of the electron packet within
layer 25 is limited to a maximum of a few million. The doping
density of layer 30 is approximately 2.times.10.sup.17 /cm.sup.3
with a thickness of 70 nanometers to provide charge necessary to
satisfy surface states.
In a preferred embodiment; substrate 10 is semi-insulating GaAs
with a thickness of 0.5 mm; Buffer layer 20 is undoped (Al,Ga)As
with 30% replacement of Gallium and a thickness of 1000 nm;
transport layer 25 is undoped GaAs with a thickness of 40
nanometers; trapping layer 30 is (Al,Ga)As with 30% replacement of
Ga, doped at 2.times.10.sup.17 /cm.sup.3, with a thickness of 70
nanometers; and cap layer 35 is undoped GaAs with a thickness of 20
nanometers.
Other combinations of semi-conductors having piezoelectric
properties are readily known to those skilled in the art and may be
substituted for the GaAs and (Al,Ga)As embodiment disclosed herein.
For example, one alternative is the use of an (Al,Ga)As buffer
layer 20 with an (In,Ga)As (with 15% Indium replacement) transport
layer 25, 20 nanometers thick. Also, InP could be used for layers
20 and 30 with (In,Ga)P (with 53% Indium replacement) as transport
layer 25. The criteria to be satisfied by a combination of
materials are the depth of the quantum well and good piezoelectric
properties coupled with a sufficiently good lattice match to the
underlying layer.
The layer 25 is known as a quantum well, and several quantum wells
may be produced by epitaxial growth techniques to increase the
charge capacity of the device. Such a structure with two quantum
wells is shown in FIG. 3. A spacer layer 22 has been added between
transport layers 25 and 25'. Spacer layer 22 is formed of
(Al,Ga)As, with 30% of the Ga replaced by Al; is undoped and has a
thickness of 100 nanometers. This method of increasing the charge
capacity is preferable to increasing the dimension of layer 25
perpendicular to the plane of the paper because the SAW wave is
limited in the transverse dimension that it can effectively
cover.
Although the invention has been shown and described with respect to
detailed embodiments thereof, it should be understood by those
skilled in the art that various changes in form and detail thereof
may be made without departing from the spirit and the scope of the
claimed invention.
* * * * *